Reladraw is a diagram language with relative placement, sitting between Mermaid's auto-layout and draw.io's manual coordinates

Sources: primary, discussion
Summary: The stated problem is that Mermaid, Graphviz and D2 decide positions for you while draw.io and Excalidraw make you pick coordinates, which the README notes is expensive for agents recalculating positions in verbose XML. Reladraw expresses placement relatively, with statements such as below, right of and level with, so a layout is stated without absolute numbers. Version 0.8.0 is Apache-2.0, with parser, layout engine and SVG renderer in TypeScript and zero runtime dependencies, and the README says the syntax is not stable yet.
Why it matters: It ships an agent skill installable across Claude Code, Codex, Cursor and Copilot, which is the reason a diagram syntax is worth noting now rather than the syntax itself.